The present invention relates to touch panels assembled from two flexible substrates each having electrically conductive traces formed by screening a conductive ink onto the conductive surfaces of the substrate.
Touch panels are described in U.S. Pat. Nos. 3,522,664; 3,591,718; 3,699,439; 3,894,183; 3,914,548; and 4,079,194 to Kley. These panels generally comprise two flexible planar membrane elements, each element being formed from a flexible plastic film coated with a thin layer of conductive material. The plastic film can be polyester film and the conductive material can be gold. The membrane elements are oriented such that the conductive coatings face each other. Operation of the touch panel is initiated by a user forcing the conductive surfaces of the elements into point contact. By alternately driving each element from a power source through a network of opposed busbars, signals are generated which represent the X and Y positions of the touch point.
A difficulty with such touch panels is non-linearity in the output of the device due to the non-uniform distribution of an electric field across the resistance, i.e., the output from a non-linear device is not directly proportional to the distance from the point activated to the busbars. For example, the outputs from two points, where one point is located twice as far from one of the busbars as the other point, do not exhibit a 2:1 response as would a linear device.
In an attempt to alleviate this problem, Kley teaches including high resistance bands parallel to the edges of the grid area to reduce shunting at the edges of the grid area.
A difficulty with the Kley device is the extra expense and labor required in producing the high resistance areas. Further, the effectiveness of the Kley device for accurately detecting changes in resistance when the touch panel is activated is questionable; there is concern that the high resistance bands will mask small variations in output resulting from small changes in the locus of the activated points. In other words, the resistance added by the high resistance bands will be so great, that the minor changes resulting from changes in where the touch panel is touched will be too small to reliably detect.
In view of these problems, there is a need for a technique to effectively and reliably linearize the output from a touch panel, without increasing the cost and difficulty in manufacturing the device.